1. Field of the Invention
The present invention concerns linear data storage devices with servo controlled head positioning. More particularly, the invention provides more accurate tape servo track following by sampling servo edges at nominal or "expected" tape head alignments established with non-servo information, and processing the samples to (1) identify optimal tape head index positions to best align data as well as servo counterparts and (2) provide corrected position error signals (PESs) for comparison during ongoing servo track following, to maintain the tape head at an optimal alignment for accurate alignment with the servo edges as well as data tracks on the tape.
2. Description of the Related Art
A data storage medium will often contain various servo signals to precisely position read/write heads with respect to tracks of data stored on the medium. A widespread example is found in magnetic tape systems, such as the IBM 3590. In IBM 3590 tape cartridges, the servo format pattern is often pre-written on the cartridge. The servo pattern includes three pairs of servo edges, each edge being a boundary between adjacent contrasting servo signals written on tape.
Nominally, the physical separation between paired servo edges is the same as the track pitch, e.g. 80 microns. The IBM model 3590 tape drive uses the locations of these servo edges to determine proper placement of written data tracks.
Drives such as the IBM model 3590 tape drive determine servo edge positions magnetically, by reading the pre-written servo patterns. Engineers have long recognized that, in most cases, servo track positioning is imperfect, and servo track pitch frequently differs slightly from the nominal track pitch of 80 microns. However, the present inventors have discovered that the variation in effective separation ("magnetic separation") between servo edges in some cases may be larger than previously thought. Physical separation between servo edges is primarily a function of the width of a physical write element of a write head used to initially lay down the servo pattern. Magnetic separation between servo edges, however, varies in response to other parameters, such as the relative amplitude of the two frequencies used to write the servo pattern. There is also an interaction between the servo read elements and the written pattern. This can cause some drives to perceive the spacing as being wider or narrower than nominal spacing.
As discovered by the present inventors, then, variability in magnetic separation between servo edges can exceed the variability in physical separation. Although servo systems in tape drives such as the IBM model 3590 amply compensate for most errors, small variations in the magnetic separation between adjacent servo edges can still permit data tracks to be written on tape with a D. C. error causing the data tracks to be squeezed together. The track mis-registration (TMR) will reduce the system read margins. In most cases, data is still written along tracks that can be easily read back by the tape head, and the data remains intact. However, in extreme cases, it is possible for a substantial error in written track placement to result in writing data with enough TMR to cause partial overwrite of an adjacent track, jeopardizing the integrity of data stored in the adjacent track. This is an especially troublesome problem in high density tape storage systems where adjacent data tracks have no intervening guard band.